Modbus Slave State Flow & Exception Codes Explained (Embedded Developer’s Guide)

This lesson explains the Modbus RTU slave state machine and the exception response mechanism from an embedded developer’s perspective.
We break down how a slave processes incoming frames, detects errors, and generates normal or exception responses based on the Modbus specification.

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In this lesson, we cover:
• The Modbus RTU slave state flow: Idle → Checking Request → Processing → Reply
• How error conditions affect the state path
• When a slave should return a normal reply vs. an exception reply
• Exception codes every embedded developer should know:
0x01 ILLEGAL FUNCTION
0x02 ILLEGAL DATA ADDRESS
0x03 ILLEGAL DATA VALUE
0x04 SLAVE DEVICE FAILURE
• Additional optional exception codes (0x05 ACKNOWLEDGE, 0x06 SERVER BUSY, etc.)
• How Modbus exception responses are formatted:
Function Code + 0x80
Exception Code
CRC
• Real exception response examples based on the slides:
07 90 01 … (Illegal Function)
07 83 02 … (Illegal Data Address)
07 83 03 … (Illegal Data Value)
07 83 04 … (Slave Device Failure)

These concepts are essential for implementing a reliable Modbus RTU slave on MCUs.

🧩 Modbus RTU Playlist (Embedded Developer Perspective):
   • Modbus RTU Explained (From an Embedded Dev...  

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